You were going OK with substituting the appropriate values into the given Fourier series. There are 3 terms given inside the last set of parentheses with the last specified term of the ongoing infinite series given as cos(6ωt)/(5x7). The fundamental frequency is ω [radians/sec]. So 6ω is the 6th harmonic. Do you need to include this in your answer?

It also remains to determine the actual value of ω, knowing that ω=2∏/T where T is the period of the waveform. You will hopefully deduce that T=16.67ms and therefore ω=???

So having determined which terms are necessary in your answer, you then need to transcribe what those terms in the series mean onto a magnitude-frequency graph. The horizontal axis should be in frequency [radians/sec will do for units or you could translate this to Hz] and the vertical axis in voltage magnitude. Each frequency component will just be a single vertical line on the magnitude-frequency graph. Notice that the first term is the average or DC value and this has "zero" frequency.